SANDSTONE CLAST BRECCIAS, HOMOGENIZED SAND, AND SAND INTRUSIONS: EVIDENCE OF SUBSTRATAL LIQUFACTION IN THE BASAL COCONINO SANDSTONE (PERMIAN), GRAND CANYON, ARIZONA

Field evidence and thin section analysis suggest the base of the Coconino Sandstone underwent substratal liquefaction prior to lithification of the unit. Previous work has indicated that the large sand-filled cracks often found at the base of the Coconino, penetrating into the Hermit Formation, are probably not mud cracks, but intrusions of liquefied sand. A number of additional features have been found at the Coconino/Hermit contact that support this hypothesis: 1) deformed and homogenized zones in the basal meter of the Coconino containing angular clasts of bedded Coconino sandstone, 2) sand-filled cracks (connected to the Coconino) which penetrate at a downward angle into the Hermit and 3) thin section analysis of the sand-filled cracks showing homogenized, non-bedded sand (typical of sand intrusions). Field observations show sand-filled cracks only occur in association with the northeast trending fault zones, which were active during the Miocene to Pliocene. Cracks are deepest near the largest faults, shorter near the smaller faults, and non-existent in places where the faults are absent. We contend the only way to explain these associated features is that the basal Coconino was water saturated and unlithified at the time of faulting. Faulting initiated liquefaction in the base of the Coconino. This resulted in homogenized zones, sandstone clast breccias, and sand intrusion emplacement into the Hermit. Features like this occur in other areas that have experienced liquefaction during tectonic activity.